Langmuir 2007, 23, 9151-9154
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Determination of Water Content in Internally Self-Assembled Monoglyceride-Based Dispersions from the Bulk Phase Anniina Salonen, Samuel Guillot,† and Otto Glatter* Institute of Chemistry, UniVersity of Graz, Heinrichstrasse 28, A-8010 Graz, Austria ReceiVed May 24, 2007. In Final Form: July 12, 2007 We determined the water intake of internally structured oil-loaded monoglyceride-based dispersions. This was possible through small-angle X-ray scattering (SAXS) experiments on the corresponding bulk mesophases because the structural parameters in full hydration conditions are identical to those of the dispersed particles. From low water contents to full hydration, the bulk phases depend strongly on the amount of oil. At room temperature in excess water and with increasing oil concentration, successive bicontinuous cubic, reverse hexagonal, micellar cubic, and inverse micellar-type isotropic fluid phases are found. The solubilized water is determined as a function of the oil content for each phase, and it is found to range from 5-33 wt %.
Introduction In recent years, lyotropic liquid crystal-forming amphiphiles have attracted interest due to their scientific and industrial applications, namely in the fields of pharmaceutics and cosmetics and in the food industry.1-4 Due to a high interfacial area and the presence of both hydrophilic and lipophilic regions, their role as carriers of active molecules has been explored.5 Some of these liquid crystalline phases can be dispersed in excess water using a stabilizer to form submicrometer size particles.6-10 Unsaturated monoglycerides11 have been widely used in the study of such dispersed particles. This family of particles, with different internal structures, have been collectively named isasomes (internally self-assembled particles or somes)9 as a generalization, grouping the emulsified microemulsion particles with cubosomes and hexosomes relating to the bicontinuous cubic and hexagonal dispersions found by Larsson and co-workers.6,7 It has been shown that the internal phase of the particles is in thermodynamic equilibrium.12 Using additives, such as oil, the internal structure of the dispersed particles can be manipulated;9,13-17 however, this can change the amount of solubilized water. From the dispersions * To whom correspondence should be addressed. E-mail: otto.glatter@ uni-graz.at. Telephone: +43 316 380 5433. Fax: +43 316 380 9850. † Present address: Centre de Recherche sur la Matie ` re Divise´e-UMR 6619, 1b rue de la Fe´rollerie, 45071 Orle´ans Cedex 2, France. (1) Mezzenga, R.; Schurtenberger, P.; Burbridge, A.; Michel, M. Nat. Mater. 2005, 4, 729-740. (2) Chang, C. M.; Bodmeier, R. J. Pharm. Sci. 1997, 86, 747-752. (3) Chang, C. M.; Bodmeier, R. J. Controlled Release 1997, 46, 215-222. (4) Esposito, E.; Menegatti, E.; Cortesi, R. J. Appl. Cosmetol. 2005, 23, 105116. (5) Chung, H.; Jeong, S. Y.; Kwon, I. C. In Bicontinuous Liquid Crystals; Lynch, M. L., Spicer, P. T., Eds.; CRC Press: Boca Raton, FL, 2005; Chapter 13. (6) Gustafsson, J.; Ljusberg-Wahren, H.; Almgren, M.; Larsson, K. Langmuir 1996, 12, 4611-4613. (7) Gustafsson, J.; Ljusberg-Wahren, H.; Almgren, M.; Larsson, K. Langmuir 1997, 13, 6964-6971. (8) Larsson, K. J. Dispersion Sci. Technol. 1999, 20, 27-34. (9) Yaghmur, A.; de Campo, L.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Langmuir 2005, 21, 569-577. (10) Spicer, P. T.; Hayden, K. L. Langmuir 2001, 17, 5748-5756. (11) Caffrey, M. Biophys. J. 1989, 55, 47-52. (12) de Campo, L.; Yaghmur, A.; Sagalowicz, L.; Leser, M. E.; Watzke, H. J.; Glatter, O. Langmuir 2004, 20, 5254-5261. (13) Yaghmur, A.; de Campo, L.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Langmuir 2006, 22, 9919-9927. (14) Popescu, G.; Barauskas, J.; Nylander, T.; Tiberg, F. Langmuir 2007, 23, 496-503. (15) Dong, Y.-D.; Larson, I.; Hanley, T.; Boyd, B. Langmuir 2006, 22, 95129518.
Figure 1. Chemical structures of (a) monolinolein (18:2) and (b) R-(+)-limonene.
themselves, it is very difficult to get the water content of the particles due to the model-dependency of the calculations, that is, assumptions on the interfacial area, internal packing, and so forth.18 It was recently proven that the structural parameters of the dispersed phases exactly correspond to those in the fully hydrated monolinolein/water bulk samples.12 Thus, it became possible to know the exact water content in each corresponding organized structure dispersed in water, which indeed is important for the solubilization of molecules,19 especially those of hydrophilic nature.20 Furthermore, it was shown that when adding another hydrophobic component, for example, n-tetradecane, to the monolinolein/water bulk system, the structural parameters in the dispersed and nondispersed phases were also similar.21 This means that the water uptake of the oil-containing particles can also be studied through the bulk phases. Nevertheless, no systematic study has been done on the water intake of the isasomes as a function of the oil content. R-(+)-Limonene (Figure 1a) is an oil widely used in industry and, as such, is of central interest as the additional hydrophobic component. Dimodan U/J is a commercial-grade form of monolinolein (Figure 1b) and has recently been used to prepare internally self-assembled limonene-loaded dispersions. The complete temperature-composition phase diagram of the particles has been realized,22 but the water content of the particles remains unknown. The phase diagram of the Dimodan U/J/water binary (16) Nakano, M.; Teshigawara, T.; Sugita, A.; Leesajakul, W.; Taniguchi, A.; Kamo, T.; Matsuoka, H.; Handa, T. Langmuir 2002, 18, 9283-9288. (17) Kamo, T.; Nakano, M.; Leesajakul, W.; Sugita, A.; Matsuoka, H.; Handa, T. Langmuir 2003, 19, 9191-9195. (18) Shearman, G. C.; Ces, O.; Templer, R. H.; Seddon, J. M. J. Phys.: Condens. Matter 2006, 18, S1105-S1124. (19) Sagalowicz, L.; Leser, M. E.; Watzke, H. J.; Michel, M. Trends Food Sci. Technol. 2006, 17, 204-214. (20) Boyd, B. J.; Whittaker, D. V.; Khoo, S.-M.; Davey, G. Int. J. Pharm. 2006, 309, 218-226. (21) Yaghmur, A.; de Campo, L.; Salentinig, S.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Langmuir 2006, 22, 517-521.
10.1021/la7015319 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/26/2007
9152 Langmuir, Vol. 23, No. 18, 2007
Letters
Figure 2. Representative SAXS curves of (a) the bicontinuous cubic Ia3hd space group (δ ) 100, 20 wt % water), (b) the bicontinuous cubic Pn3hm space group (δ ) 100, 35 wt % water), (c) the hexagonal phase HII (δ ) 95, 35 wt % water), and (d) the micellar cubic Fd3hm space group (δ ) 60, 20 wt % water). The insets show the 2D scattering pattern before vertical integration of the intensity, where the differences in the images arises from different detectors used: (a-c) CCD and (d) image-plate.
system has been determined by Mezzenga et al.23 They found a strong decrease in the maximum amount of solubilized water with temperature. In this paper, we present the water content in the R-(+)limonene/Dimodan U/J isasomes at room temperature as a function of their composition through the exploration of the bulk phase behavior. This will allow for further models on the arrangement of the internal structures, as well as understanding on the solubilization capabilities of such particles. Experimental Section Dimodan U/J (DU) was supplied by DANISCO A/S (Braband, Denmark). It contains 96% distilled monoglycerides, of which 62% are linoleate. R-(+)-Limonene was purchased from Fluka (purity > 96%). For reasons of clarity, the term oil is used to refer to R-(+)limonene in the following. The dispersions discussed were stabilized using the triblock copolymer Pluronic F127 (a gift from BASF Corporation, Mount Olive, NJ). The materials were used without any further purification. The water used in the preparations was double distilled. DU/oil mixtures were prepared at different oil contents, where the balance between DU and oil is defined as the weighted DU/oil mass ratio δ)
MDU 100 [%] MDU + Moil
(22) Guillot, S.; Moitzi, C.; Salentinig, S.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Colloids Surf., A. 2006, 291, 78-84. (23) Mezzenga, R.; Meyer, C.; Servais, C.; Romoscanu, A. I.; Sagalowicz, L.; Hayward, R. C. Langmuir 2005, 21, 3322-3333.
After weighing the DU/oil mixture and water into Pyrex tubes, the samples were heated using a Bunsen burner (up to 100 °C for a few seconds at a time) with intermittent vigorous mixing using a vortex for thorough homogenization. They were then left for cooling for 2 h at room temperature (25 °C). Afterward, they were frozen (-4 °C) for 1 h before subsequent re-equilibration at room temperature (1 week) and small-angle X-ray scattering (SAXS) measurements at 25 °C. Concerning the dispersions, the stabilizer to the DU/oil ratio was kept constant at 8.1% and the weight fraction of the dispersed phase (DU/oil + stabilizer) was always 5 wt %, with 95 wt % water. The raw mixture was ultrasonicated (SY-Lab GmbH, Pukersdorf, Austria), without external cooling, for 20 min at 30% of maximum power in pulse mode (0.5 s on and 1.5 s off), and further details are given in ref 22. The SAXS equipment consists of a slit-geometry camera (SAXSess, Anton-Paar, Austria) connected to an X-ray generator (Philips, PW1730/10) operating at 40 kV and 50 mA with a sealedtube Cu anode (λ ) 0.154 nm). The 2D scattering pattern is recorded by an imaging-plate detector or by a charge-coupled device (CCD) camera (examples are shown in the insets of Figure 2). Details of all setups are given in ref 22. The images are integrated into the one-dimensional scattering function I(q) (see Figure 2). The temperature of the capillary in the metallic sample holder is controlled by a Peltier element. A thermal equilibration time of 30 min was used before each SAXS measurement, and the measuring times were 3 × 5 min with the CCD camera and 10 min with the imagingplate detector.
Results and Discussion The structural determination of the bulk phases will enable us to deduce the water content in the isasomes. We start by presenting the characterization of the different SAXS patterns observed in the system (Figure 2).
Letters
Figure 3. Lattice parameter and characteristic distance evolution of Dimodan U/R-(+)-limonene/water bulk phases with increasing water content for δ ) 50, 65, 95, and 100. The lines shown are linear fits to the data, and the maximum water solubilization is determined as the crossover of the two behaviors.
In the binary DU/oil mixture, two different bicontinuous cubic phases are found. At intermediate water contents, the scattering pattern shows eight Bragg peaks with relative positions in the ratios x6, x8, x14, x16, x20, x22, x24, and x26 (Figure 2a). These can be indexed as hkl ) 211, 220, 321, 400, 420, 332, 422, and 431 reflections of the bicontinuous Ia3hd space group. At higher water contents, the scattering exhibits at least seven peaks with the relative ratios of x2, x3, x4, x6, x8, x9, and x10 (Figure 2b). These peaks are indexed as hkl ) 110, 111, 200, 211, 220, 221, and 310 reflections of another bicontinuous cubic lattice of Pn3hm symmetry. Adding oil to the system promotes more negative interfacial curvatures (i.e., higher curvature toward the water region) than bicontinuous cubic symmetries (i.e., an inverse hexagonal symmetry). The effect of added oil can also be discussed in terms of the hydrocarbon volume V, critical chain length lc, and optimal surface area of the headgroup a0, which are combined in the packing parameter defined as V/(a0lc). Therefore, as the amount of oil is increased, V increases, promoting higher packing parameters and thus a more negative curvature of the monolayer. Indeed, in this case, we observe scattering patterns with five Bragg peaks with relative positions in the ratios 1, x3, x4, x7, and x9, which can be identified as a hexagonal phase HII (Figure 2c). Further addition of oil results in a scattering pattern with at least nine peaks (Figure 2d) in the ratios of x3, x8, x11, x12, x16, x19, x24, x27, and x32, which are indexed as hkl ) 111, 220, 311, 222, 400, 331, 422, 333 + 511, and 440 reflections of an inverse micellar cubic phase of the Fd3hm symmetry. Finally, at sufficiently low δ, we measure a broad peak characteristic of an L2 phase (not shown here), where a characteristic distance d ) 2π/q0 can be deduced from the maximum of the broad correlation peak at q0. Now, we can proceed with the detailed description of the effect of increasing water at different δ-values. At δ ) 100 and at water contents below 20 wt %, lamellar phases and inverse micellar solutions are found. This is beyond the scope of this paper and will not be further described. In the range of 20-30 wt % water, the system is an Ia3hd cubic, while above 35 wt % a transition into a pure Pn3hm is seen. At 30 wt % water, a mixture of these two cubic phases is obtained. The evolution of the lattice parameter in these phases is shown in Figure 3. In the Ia3hd regime, the lattice parameter increases with the water content, so the system is not at saturation, but it is constant in the pure
Langmuir, Vol. 23, No. 18, 2007 9153
Figure 4. Phase diagram of the ternary Dimodan U/R-(+)-limonene/ water system at 25 °C showing the maximum solubilization line of water (b). The dotted lines that separate the phases were drawn using the average values between the measured points.
Pn3hm regime. Thus, the maximal water content without oil is in the range of 30-35 wt % water, in agreement with literature values.20 Generally, upon increasing the water content (without a change of phase), the scattering peaks shift to lower q-values with increasing water until a limit at which the peak positions remain identical independent of water content is reached. This means that the lattice parameter increases with increasing water until saturation. Indeed, above this saturation, the samples become turbid, a characteristic of excess water. Therefore, an increase in the amount of water leads to fuller hydration of the monoglycerides and to an increase in a0; thus, this leads to a smaller negative curvature. Adding some oil (75 e δ e 95), the phase sequence when increasing the water content is inverse micellar solution (
